Sealing structure for display devices

ABSTRACT

The invention relates to a sealing structure ( 6 ) suitable for a polymer-based electronic device ( 1 ), charaterized in that the sealing structure ( 6 ) comprises a first layer ( 7 ) of a first dielectric material formed on said device ( 1 ) and a second layer ( 8 ) of a second dielectric material formed on the first layer ( 7 ).

The present invention relates to a sealing structure suitable fordisplay devices, and to a method of sealing the device.

Display devices are well known in graphic display and imaging art.Examples of such devices are organic light emitting devices (OLED) andpolymer light emitting devices (PLED), and liquid crystal displays(LCD), which have practical application for TV and graphic displays, aswell as in digital printing applications.

Most of these devices are sensitive to oxygen and moisture etc. and,consequently, degrade when exposed to the atmosphere. Exposure to oxygenand/or moisture, particularly in the presence of light, may lead tophoto-oxidative degradation of a polymer material used. Furthermore,oxidation at the cathode/polymer interface is one of the first problemsarising with oxygen and/or moisture diffusion in devices such as OLEDsor PLEDs. Such reactions will significantly reduce the performance ofthe light emitting properties of the device. Moreover, other materialsused in the display device may also degrade when exposed to theatmosphere; they may be negatively affected by, for example, oxidation.

Therefore, there is a need for an encapsulation of the device. Effortshave been made to evaporate metal films so as to seal display devices.However, these films often contain pinholes, and therefore must berelatively thick, which results in poor light transmission. Many of theknown processes for achieving a more or less adequate encapsulation ofthe devices require temperatures above 300° C. during the encapsulationprocess. Most polymer-based devices are not compatible with such hightemperatures.

WO 00/08899 discloses a light-emitting device with a sealing structureapplied by a low-temperature technique encapsulating the device. Thissealing structure comprises a non-reactive material, such as aluminium,and a thin film of an inorganic refractory material, as an outermostlayer. This structure prevents oxygen and/or moisture from reacting withthe materials in the device. However, this sealing structure hasproblems concerning the non-reactive material, which normally is ametallic film. The metallic film is a conductor, which increases therisk of electronic short-circuits between the electrodes in the deviceand the metallic film. Moreover, the metallic film in the sealingstructure imposes another problem concerning the texture of theelectrode material. Sometimes it is necessary to have some kind oftexture having negative slopes, which also have to be covered with thesealing structure. However, metals do not normally deposit on suchslopes, thus leaving a partly closed surface. In turn, this may allowoxygen and/or moisture to penetrate into the device.

Consequently, the object of the present invention is to provide anencapsulation which overcomes the above problems of the prior art.

These and other objects are achieved by a sealing structure which ischaracterized in that the sealing structure comprises a first layer of afirst dielectric material and a second layer of a second dielectricmaterial. No short-circuits can occur in this structure. Furthermore,making the metal lids superfluous reduces the production costs.Moreover, by using at least two layers of dielectric materials, apinhole free sealing structure can be obtained and also an even texturehaving negative slopes for the entire surface of the display device.

Moreover, the sealing structure is suitably transparent and preferablycomprises at least one layer of a third dielectric material formed ontop of the second layer; the outer layer is then made of a materialresistant to the atmosphere.

In accordance with a preferred embodiment of the invention, said firstand the third dielectric materials are the same, and are preferablysilicon nitride. The second dielectric material is selected from amongsilicon oxide, silicon oxynitride, silicon oxidefluoride, titaniumoxide, tantalum oxide, zirconium oxide, hafnium oxide, aluminium oxide,or any mixture thereof.

The sealing structure of the present invention may also comprise agetter layer which functions as a built-in moisture trap inmoisture-sensitive devices.

Moreover, the sealing structure may also comprise a layer of an organicpolymer between two layers of dielectric materials, and preferably agetter layer is formed on top of the layer of the organic polymer.

The invention also relates to a method for sealing an polymer-basedelectronic device, comprising the steps of forming a first layer of afirst dielectric material on said electronic device and of forming asecond layer of a second dielectric material on top of the first layer.The method also comprising a step of forming at least one layer of athird dielectric material on top of the second layer.

Finally, the present invention relates to a display device, comprising asubstrate and at least one sealing structure according to the presentinvention. The sealing structure may be applied either on the sidefacing a potential user, or between the substrate and the display deviceto prevent diffusion of oxygen and/or moisture through the substrate.Moreover, a sealing structure according to the invention may be appliedon both sides of the display device.

A preferred embodiment comprises on a flexible substrate an anode and atransparent cathode, in between of which an organic electroluminescentlayer is present. The anode preferably comprises a reflective metal,such as Ag, Pt or Au. In this manner, a flexible display is providedwherein the light is not provided through the anode and the substrate,but through the cathode and the transparent and flexible sealingstructure of the invention. Preferred materials for the transparentcathode include indium-tin-oxide (ITO) andpoly-3,4-ethylenedioxythiophene (PEDOT).

Further preferred embodiments of the invention are described in theremaining dependent claims.

Preferred embodiments of the present invention will be described indetail hereinafter, by way of non-limitative example, with reference tothe accompanying drawing.

FIG. 1 is a cross-sectional side view of an embodiment of a displaydevice of the present invention, and

FIG. 2 is a cross-sectional side elevation another embodiment of thepresent invention.

The present invention relates to a sealing structure suitable fordisplay devices, such as OLEDs, PLEDs and LCDs.

FIG. 1 shows a PLED display device including a sealing structureaccording to the present invention. The device 1 is a layered structureof a light-emitting unit, comprising an anode 2, a cathode 3 and alight-emitting layer 4 interposed between the anode 2 and the cathode 3.The light-emitting layer 4 may be selected from among a multitude ofluminescent materials, depending on the desired light emittingcharacteristics, and the layer 4 may consist of a multitude ofsub-layers. Suitable materials for the anode 2 are transparentconducting thin films of preferably indium-tin-oxide which enable thelight to be emitted by the device at useful levels. The cathode 3 isusually not transparent and comprises, for example, Ba and Al, or LiFand Al. For some applications transparent cathodes are desired, andthen, for example, indium-tin-oxide may be used. However, thosematerials do not limit the present invention in any way and othermaterials known to those skilled in the art may be used. When apotential difference is applied across the device, light is emitted andthe generated color depends on the electronic properties of theluminescent material. Depending on the composition of the materialmaking up the light-emitting layer, many different colors of light maybe produced. In the display device 1 it is possible, in conformity withthe invention and the properties of anodes and cathodes, to interchangethe anode 2 and the cathode 3.

The above embodiment of the display device 1 is arranged on a substrate5. The substrate 5 is often a soda-lime or boron-silicate glass;however, other, more or less oxygen and/or moisture permeable substratesmay be used, for example, plastic and/or flexible substrates. Moreover,the substrate maybe an opaque or a clear substrate. The size, shape andmaterial of the substrate 5 may vary in dependence on the intended useof the device 1. The layered structure of the anode 2, the lightemitting layer 4, and the cathode 3 is formed on the substrate 5 usingknown manufacturing methods, for example, sputtering techniquesevaporation techniques and various deposition techniques such aschemical and physical vapor deposition. The appropriate technique to beused depends on the material used for the different layers and will beknown to a person skilled in the art.

A sealing structure 6, functioning as a diffusion barrier to oxygenand/or moisture, is formed on the display device, and hence a muchlonger durability is achieved.

In this embodiment of the present invention the sealing structure 6consists of three layers 7,8,9 of dielectric materials which arepreferably transparent materials. The layer 7 should preferably be oflow pinhole density and should cover substantially the entire surface ofthe cathode 3. The layer 7 of a first dielectric material is formedafter the deposition of the cathode 3 without exposing the displaydevice 1 to air, that is, while keeping the device 1 in an inertatmosphere. On top of the layer 7, a layer 8 of a second dielectricmaterial is formed while using preferably the same technique as for thelayer 7. Finally, an outer layer 9 of a third dielectric material isformed on top of the layer 8 by essentially the same technique.

The layer 7 preferably comprises silicon nitride which closes most ofthe area of the device 1 and leaves only a few pinholes open. However,these pinholes would not be closed by making the silicon nitride layerthicker, since the chemical surface of the pinholes prevents sticking ofthe nitride. By depositing the layer 8, which preferably comprises anoxide, the surface of the layer 7 is modified and the oxide layer 8 willcover the pinholes. However, the oxide layer is generally not asignificant diffusion barrier for oxygen and/or moisture as siliconnitride is. Therefore, the layer 9 is deposited on top of the layer 8and comprises silicon nitride. A N-O-N sealing structure is thusachieved, where N refers to a nitride layer and O to an oxide layer.

A wide range of oxides may be used for the oxide layer 8, and preferablyis selected from among silicon oxide, silicon oxynitride, siliconoxidefluoride, titanium oxide, tantalum oxide, zirconium oxide, hafniumoxide, aluminium oxide, or any mixture thereof. Among these materialssilicon oxide has shown surprisingly good results and hence is to bepreferred.

A low temperature plasma-enhanced CVD (PECVD) method has been used forthe formation of the layers 7,8,9. However, other techniques known tothose skilled in the art may be used in dependence on the materialsused. The sealing structure 6 of the present embodiment comprises about200 nm of silicon nitride, about 300 nm of silicon oxide and about 200nm of silicon nitride, deposited at about 80° C. In the case of thissealing structure there is no need for a metallic film between thedisplay device and the dielectric material and, consequently, noshort-circuits will occur. Moreover, the production costs are reduced bymaking the metal lids superfluous.

The N-O-N sealing structure according to the present invention has awater permeation rate of less than 1.10⁻⁶ g/m²/day, which may beimproved, without changing the materials, by just adjusting thethickness. Moreover, more than 80% of the visible light spectrum istransmitted. For a N-O-N-O-N sealing structure about 80% of the light ofa wavelength of 470 nm and more than 80% for the other part of thevisible spectrum is transmitted. Moreover, this sealing structure mayalso function as an ultraviolet-blocking layer, thereby protectinglight-sensitive materials in the device against UV exposure.

The layer thicknesses of the N-O-N and the N-O-N-O-N sealing structuresof the invention may vary. Next to a 300-200-300 nm stack good resultshave been obtained with a stack of 200-100-100-100-100-100 nm stack,which stack was deposited at 130° . The thickness may be reduced to 50nm per layer and it is preferred that the first layer has a somewhatlarger thickness.

FIG. 2 shows another embodiment of the present invention. In thisembodiment, the display device 1 comprises a flexible, plastic substrate5 and at least one sealing structure 6 of dielectric materials asdescribed above. Since oxygen and/or moisture may penetrate through theflexible substrate 5, a sealing structure 6 according to the presentinvention is interposed between the substrate 5 and the light-emittingunit 1. The invention is not limited to flexible, plastic substrates,and other substrates may be used. Furthermore, the device may also beencapsulated by applying a sealing structure 6 on the side of thedisplay device facing a potential user. Using this sealing structure 6on a flexible substrate, it has been demonstrated that more than 80% ofthe light is transmitted, that is, both visible light and at 470 nm.

Another embodiment of the present invention has a getter layer includedin the sealing structure. This getter layer is formed as an additionallayer in the sealing structure and may be used irrespective of thenumber of dielectric layers in the sealing structure. This getter layerfunctions as a built-in moisture-trap for moisture sensitive devices. Ina sealing structure of N-O-N, or in a sealing structure of O-N-O, thegetter layer must preferably be formed on top of the oxide layer. Thisis because an inclusion of oxygen in the CVD process might reduce theefficiency of the getter material. The getter material is preferablychosen so that it does not react with the formulation of the next layerdeposited in the sealing structure, and preferably especially so that noreactions take place with nitrogen in the formation of silicon nitridelayers. Examples of suitable materials are BaO and CaO. Moreover,metallic calcium reacts only slowly with nitrogen and may also be used.

According to another embodiment of a sealing structure of the presentinvention, it is possible to form a layer of an organic polymer betweentwo layers of dielectric materials. The dielectric materials used arepreferably an oxide or nitride of silicon; however other materials maybe used. A getter layer may also be formed, that is, preferably on topof the layer of organic polymer and before the formation of the nextlayer of dielectric material. However, using a organic polymer incombination with a getter layer, the sealing structure preferablycomprises at least one nitride layer, but more preferably more than onenitride layer.

The present invention should not be considered as being limited to theabove-described embodiments, but rather includes all possible variationscovered by the scope defined by the appended claims. The sealingstructure may be used in any situation where an oxygen and/or moisturesealing structure is needed.

Moreover, the present invention is not limited to a sealing structure ofN-O-N; other sealing structures of N-O-N-O-N and N-O-N-O-N-O-N etc. mayalso be used. A sealing structure of O-N-O may also be used.Furthermore, other dielectric materials may also be used. The sealingstructure may also comprise a multitude of alternating layers oftransparent dielectric materials. The structure is believed to provideunexpectedly good results in that any microcracks or pinholes will bepresent at a first location in the first layer, and, due to thedifferent constitution of the second layer, at a different location andwith different texture in that layer. In any third layer these will beagain at another location. In this manner, there is virtually nocontinuous path through the sealing structure.

1. A sealing structure (6) for a display device (1), characterized inthat the sealing structure (6) comprises a first layer (7) of a firstdielectric material formed on said device (1), a second layer (8) of asecond dielectric material formed on the first layer 97), at least onethird layer of a third dielectric material is formed on top of thesecond layer, and a getter layer, wherein said sealing structurecomprise an N-O-N- or O-N-O- structure and the getter layer is formed ontop of one of the at least one O layers.
 2. A sealing structureaccording to claim 1, wherein said dielectric materials are transparent.3. A sealing structure according to claim 1, wherein at least an outerlayer (8, 9) facing a potential user comprises a dielectric materialresistant to the atmosphere.
 4. A sealing structure according to claim1, wherein the first and the third dielectric materials are essentiallythe same.
 5. A sealing structure according to claim 4, wherein the firstand the third dielectric materials are silicon nitride.
 6. A scalingstructure according to claim 1, wherein the second dielectric materialis selected from among silicon oxide, silicon oxynitride, siliconoxidefluoride, titanium oxide, tantalum oxide, zirconium oxide, hafniumoxide, aluminium oxide, or any mixture thereof.
 7. A method for sealinga display device (2), comprising the steps of: forming a first layer (7)of a first dielectric material on said display device (1), forming asecond layer (8) of a second dielectric material on top of the firstlayer (7), and forming at least one third layer of a third dielectricmaterial on top of the second layer and a getter layer and wherein saidfirst, second and third layers form an N-O-N- or O-N-O- structure andthe getter layer is formed on top of one of the at least one O layers.8. A method according to claim 7, wherein the first and the thirddielectric material are the same.
 9. A method according to claim 7,wherein said layers (7, 8, 90 are formed using a low-temperatureplasma-enhanced CVD method.
 10. A display device (1) comprising asubstrate (5) and at least one sealing structure (6) according toclaim
 1. 11. A display device according to claim 10, wherein the atleast one scaling structure is at least formed on the side of thedisplay device (1) facing a potential user.
 12. A display deviceaccording to claim 10, wherein the at least one sealing structure isinterposed between the display device and the substrate (3).